This invention relates to combustion noise dampers for use in burner assemblies.
A particularly troublesome characteristic of combustors is combustion screech, a persistent audible noise having a substantial amount of its acoustic power concentrated in a fundamental frequency and its overtones, all of which depend primarily upon the length of the combustor housing, generally a cylinder having a closed end at which a burner is mounted and an open end through which combustion products exit from the combustor.
A combustor housing typically is closed at one end and open at the other. The burner nozzle assembly is ordinarily mounted adjacent the closed end. In the burner components, combustion-supporting and promoting materials are mixed and injected into the combustor housing. An igniter ignites these, and a flame is established at or adjacent the closed end of the housing. The flame is the source of the combustion screech's acoustic power. The closed end of the housing is a displacement node and a pressure antinode of the screech waveform. The open end of the housing is at approximately a pressure node and a displacement antinode of the screech waveform. If the length of the housing is considered to be L, it will be appreciated that another displacement antinode and pressure node will lie at a distance approximately 1/3 the length L of the housing, measured from the closed end.
An organ pipe provides a simple study of sound originating in a vibrating air column. If both ends of a pipe are open and a stream of air is directed against an edge, standing longitudinal waves can be set up in the pipe. The air column will then resonate at its natural frequencies .omega..sub.n of vibration, given by ##EQU1## where v is the speed of the longitudinal waves in the column whose superposition can be thought of as giving rise to the vibrations, and n is the number of half wavelengths in the length L of the column. The fundamental and overtones are excited at the same time. In an open pipe the fundamental frequency corresponds approximately to a displacement antinode at each end and a displacement node in the middle. Hence, in an open pipe the fundamental frequency is v/2 L and all harmonics are present. In a closed pipe the closed end is a displacement node. The fundamental frequency is approximately v/4 L, which is one-half that of an open pipe of the same length. The only overtones present are those that give a displacement node at the closed end and an antinode (approximately) at the open end. Hence, the second, fourth, etc., harmonics are missing. In a closed pipe, odd harmonics are present. Although the open end of a combustor typically cannot be considered particularly narrow compared to the length of the sound wave produced therein (one of the assumptions in most elementary analyses of standing waves in air columns), generally, a displacement antinode of the sound wave will still lie fairly close to the open end of the combustor housing.
According to the invention, a combustor comprises a housing having a generally cylindrical sidewall, a first end and a second, open end. A nozzle assembly mixes combustion supporting materials in appropriate proportions to support combustion. The nozzle is mounted adjacent the first end. Means are provided for igniting the mixed combustion supporting materials. At least one opening is provided through the sidewall between the first end and the second end.
Illustratively, according to the invention, there are multiple openings and the largest cross sectional dimension of each of the openings is less than about five percent of the largest cross sectional dimension of the second end. Illustratively, the largest cross sectional dimension of each of the openings is less than or equal to about two percent of the largest cross sectional dimension of the second end.
Further, illustratively, the openings are spaced at substantially equal intervals around the sidewall transverse to a longitudinal extent of the sidewall.
Additionally, illustratively, the sidewall is generally right circular cylindrical.
Additional objects, features, and advantages of the invention will become apparent to those skilled in the art upon consideration of the following detailed description of preferred embodiments exemplifying the best mode of carrying out the invention as presently perceived.